You are a brick

C.S. Lewis の小説 The Magician's Nephew を読んでいます。
"Now's my chance," thought Digory. He darted between the horse and the railings and began going forward. If only the brute would stay still for a moment he might catch the Witch's heel. As he rushed, he heard a sickening crash and a thud. The Witch had brought the bar down on the chief policeman's helmet: the man fell like a nine-pin.

"Quick, Digory. This must be stopped," said a voice beside him. It was Polly, who had rushed down the moment she was allowed out of bed.

"You are a brick," said Digory. "Hold on to me tight. You'd have to manage the ring. Yellow, remember. And don't put it on till I shout."
"You are a brick" と言われても "brick" の意味はレンガしか知りません。辞書を見ます。
・Oxford English Dictionary: British informal, dated A generous, helpful, and reliable person.: “You are really a brick, Vi,” Gloria said.

・Wiktionary: A helpful and reliable person.: Thanks for helping me wash the car. You're a brick.

・Cambridge English Dictionary: a very helpful and kind person who can be trusted: Thanks for bringing all that food along to the party, Tony. You're a brick!
イギリス英語でしかも古い表現の様ですね。子供向けの話ですが、米国の子供たちもこの表現が直ぐに分かるのでしょうか？